Chemical Abundances of the Damped Lya Systems at z>1.5

Abstract

We present chemical abundance measurements for 19 damped Lya systems observed with the high-resolution echelle spectrograph (HIRES) on the 10 m W.M. Keck Telescope. We perform a detailed analysis of every system, deriving ionic column densities for all unblended metal-line transitions. Our principal goal is to investigate the abundance patterns of the damped systems and thereby determine the underlying physical processes that dominate their chemical evolution. We place particular emphasis on gauging the relative importance of two complementary eects often invoked to explain the damped Lya abundances (1) nucleosynthetic enrichment from Type II supernovae and (2) an interstellar mediumÈlike (ISM-like) dust-depletion pattern. Similar to the principal results of Lu et al., our observations lend support both for dust depletion and Type II supernova (SN) enrichment. SpeciÐcally, the observed over-abundance of Zn/Fe and underabundance of Ni/Fe relative to solar abundances suggest signiÐcant dust depletion within the damped Lya systems. Meanwhile, the relative abundances of Al, Si, and Cr versus Fe are consistent with both dust depletion and Type II supernova enrichment. Our measurements of Ti/Fe and the Mn/Fe measurements from Lu et al., however, cannot be explained by dust depletion and indicate an underlying Type II SN pattern. Finally, the observed values of [S/Fe] are inconsistent with